Alkali Content as a Tool for Tailoring ZSM-48 Physicochemical Properties: From Crystallization Kinetics to Catalytic Performance in n-Hexadecane Hydroisomerization

The morphology and pore structure of ZSM-48 zeolite are critical parameters determining the catalytic performance of bifunctional catalysts in the hydroisomerization of long-chain n-paraffins. This study investigates the effects of the Na2O/SiO2 molar ratio (0.02–0.12) in the synthesis gel and hydrothermal treatment duration (48–72 h) on the crystallization kinetics, phase purity, and physicochemical properties of ZSM-48. Low alkalinity (Na2O/SiO2 = 0.04–0.06) and shorter synthesis times (48 h) promote the formation of small aggregates composed of short needle-like crystals with enhanced intercrystalline mesoporosity. Conversely, increasing the alkalinity and crystallization duration accelerates crystal growth, resulting in dense pseudo-spherical aggregates (up to 4–7 μm in size) with restricted external surface area and increased diffusion limitations. Catalytic testing of Pt/ZSM-48 (0.5 wt.% Pt) in n-hexadecane hydroisomerization demonstrates that crystal morphology, size, and porosity significantly influence process selectivity. The catalyst based on nanosized ZSM-48 (Pt/Z48-06-2) effectively mitigates diffusion resistance, yielding a maximum isomer yield of 73% at 82% selectivity. In contrast, larger, densely packed aggregates with high but poorly accessible acidity intensify secondary hydrocracking reactions, reducing a maximum isomer yield to 46%. These results highlight the ability to tune the catalytic properties of ZSM-48 through careful control over gel alkalinity and crystallization kinetics.